The present invention relates to phosphors containing oxides of alkaline-earth and Group-IIIB metals activated with rare-earth ions. In particular, the present invention relates to phosphors containing oxides of alkaline-earth and Group-IIIB metals activated with at least europium ions wherein the ratio of positive ions and negative ions is off-stoichiometric. The present invention also relates to light sources incorporating such phosphors.
A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called xe2x80x9cactivatorsxe2x80x9d) have been added to convert them into efficient fluorescent materials. With the right combination of activators and inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range. Well-known phosphors have been used in mercury vapor discharge lamps to convert the ultraviolet (xe2x80x9cUVxe2x80x9d) radiation emitted by the excited mercury vapor to visible light. Other phosphors are capable of emitting visible light upon being excited by electrons (used in cathode ray tubes) or x rays (for example, scintillators in x-ray detection systems).
The efficiency of a lighting device that uses a phosphor increases as the difference between the wavelength of the exciting radiation and that of the emitted radiation narrows. Therefore, in the quest for improving efficiency of white light sources, effort has been dedicated to finding a source of stimulating radiation that has wavelengths in the range of near-UV to blue visible light and phosphors that respond to those wavelengths. The term xe2x80x9cnear-UVxe2x80x9d means UV radiation having wavelengths in the range from about 315 nm to about 400 nm. Recent advances in light-emitting diode (xe2x80x9cLEDxe2x80x9d) technology have brought efficient LEDs emitting in the near-UV-to-blue range. These LEDs emitting radiation in the near UV-to-blue range will be hereinafter called xe2x80x9cUV/blue LEDs.xe2x80x9d As used herein, a UV/blue LED may emit radiation having wavelengths in the near UV range, in the blue light range, or in a broad range from near UV to blue. It would be an advance to the technology of lighting to provide a range of phosphors that can be stimulated by the radiation emitted from these UV/blue LEDs radiation sources to allow for a flexibility in the use of phosphors for generating various color LEDs. Such phosphors when combined with the emission from the UV/blue LEDs can provide efficient and long lasting lighting devices that consume little power.
Many near UV/blue LEDs based on combinations of nitrides of indium, aluminum, and gallium have recently appeared. For example, U.S. Pat. No. 5,777,350 disclosed LEDs comprising multiple layers of indium and gallium nitrides and p- and n-type AlGaN, which emit in the wavelength range of about 380 nm to about 420 nm. The active layer of such a LED may be doped with other materials to shift the LED peak emission within the UV-to-blue wavelength range. A LED having a peak emission in the blue light wavelengths was combined with a coating of a yellow light-emitting yittrium aluminum garnet phosphor activated with cerium (xe2x80x9cYAG:Cexe2x80x9d) to produce white light is disclosed in U.S. Pat. No. 5,998,925. Although a substantial portion of the need for white light devices may be filled by LED-based devices, the ability to combine a UV/blue LED with a phosphor has been limited because YAG:Ce has been the only known yellow light-emitting phosphor that is excitable by radiation in the blue range.
White light can also be generated by mixing blue, green, and red light. Therefore, it is desirable to provide novel efficient phosphors that are excitable in the near UV-to-blue range and emit in these colors. It is especially desirable to provide efficient green-emitting phosphors because well-known green-emitting phosphors are largely excitable in the mid-UV wavelength range (about 200-300 nm). It is also desirable to provide novel phosphor blends that can be combined with UV/blue LEDs to produce white light of high efficiency and/or high color rendering index (xe2x80x9cCRIxe2x80x9d).
The present invention provides phosphors containing oxides of alkaline-earth and Group-IIIB metals activated with rare-earth ions activated with rare-earth metal ions that are excitable by radiation having wavelengths in the near UV-to-blue range (from about 315 nm to about 480 nm) to emit efficiently in the range of green-to-yellow visible light from about 480 nm to about 600 nm. In general, the phosphors of the present invention are oxides of alkaline-earth and Group-IIIB metals doped with at least europium ions that act as an activator. One or more other rare-earth metal ions can be included as a co-activator, which other rare-earth metal ions are selected from the group consisting of ions of cerium, praseodymium, neodymium, samarium, gadolinium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium: The phosphor of the present invention has a general formula of
(M1-xREx)yD2O4;
wherein M is an alkaline-earth metal selected from the group consisting of Sr, Ba, Ca, and combinations thereof; RE is a rare-earth metal comprising at least europium; D is at least a metal of Group IIIB of the Periodic Table selected from the group consisting of aluminum, gallium, indium, and combinations thereof; 0.001 less than x less than 0.3, and y satisfies a condition selected from the group consisting of 0.75 less than y less than 1 and 1 less than y less than 1.1.
In one aspect of the present invention, the phosphor can further comprise magnesium.
In another aspect of the present invention, a method for producing a phosphor comprising oxides of at least an alkaline-earth metal and at least a Group-IIIB metal activated with at least europium, which method comprises the step of: (a) providing amounts of compounds of: at least a rare-earth metal comprising at least europium; at least an alkaline-earth metal selected from the group consisting of strontium, barium, calcium, and combinations thereof; and at least a Group-IIIB metal selected from the group consisting of aluminum, gallium, indium, and combinations thereof; the amounts being chosen such that the final composition of the phosphor is achieved; (b) optionally adding at least one fluxing compound selected from the group consisting of halides of at least a metal selected from the group consisting of strontium, barium, calcium, aluminum, gallium, indium, and rare-earth metals; (c) mixing together the compounds; and (d) firing the mixture in a reducing atmosphere at a temperature and for a time sufficient to convert the mixture to a phosphor comprising oxides of at least an alkaline-earth metal and at least a Group-IIIB metal activated with at least europium.
In still another aspect of the present invention, the method further comprises the step of converting the compounds of the mixture into oxygen-containing compounds before firing in the reducing atmosphere.
In still another aspect of the present invention, a light source emitting visible light comprises a UV/blue LED and at least said phosphor comprising oxides of at least an alkaline-earth metal and at least a Group-IIIB metal activated with at least europium.
Other features and advantages of the present invention will be apparent from a perusal of the following detailed description of the invention and the accompanying drawings in which the same numerals refer to like elements.